1. Field of the Invention
The present invention relates to an oxide superconductor including barium (Ba) at least, and to a process for producing it. The oxide superconductor prepared by the present invention has a high critical current density, and is an extremely useful material for a superconducting wire, a superconducting tape and other applications.
2. Description of Related Art
In 1986, it was discovered that La-Ba-Cu-O oxide exhibited a sharply dropped electric resistance at around 30K. Since the discovery, the research and development on oxide superconductors have been carried out very actively. At present, Y-Ba-Cu-O oxide is known as a high temperature superconductor which has high critical temperature of around 90K, and it is investigated for many applications.
When making a superconducting magnet, it is required to make the superconductor into a wire or a tape. Accordingly, the superconductor has been first filled in a sheath comprising copper (Cu), silver (Ag) or the like. Then, the sheath filled with the superconductor is swaged and rolled, and thereafter it is heat treated.
Despite its name, a superconductor, it is impossible to flow a current over a certain critical value. The critical value is critical current density (hereinafter abbreviated to "Jc"). The "Jc" plays the most important role for characterizing the superconductor for an actual applications. However, other than superconductors formed in thin films, no "Jc" at a practical level value has not been attained so far. Therefore, an improved "Jc" value has been longed for a superconductor formed in a wire or a tape.
The presence of the grain boundaries and the magnetic flux pinning force has been known as causes which influence the "Jc" value greatly. Namely, it has been known that the presence of the grain boundaries causes to decrease the "Jc" value in polycrystalline sintered bodies. Further, it has been also known that precipitated particles work as pinning centers and improve the "Jc" value.
Accordingly, there has been proposed recently a production process, which relates to a melting method or the like, as set forth in an extra issue of a magazine, "Yoyu-En (Molten Salt)" Vol. 32, No. 3. The production process eliminates the grain boundaries from a Y-Ba-Cu-O oxide, and disperses the precipitates produced by melting and decomposing in the Y-Ba-Cu-O oxide. Since the Y-Ba-Cu-O oxide has no grain boundaries, and since the precipitates produced by melting and decomposing work as the pinning centers of magnetic flux, a high "Jc" value can be obtained by the production process.
According to the production process, i.e., a melting method as described above, an oxide powder is first heated at a high temperature, and thereafter the molten oxide is cooled rapidly so as to decompose the compositions. Then, precipitation phases to be the pinning centers and superconductor phases having no grain boundaries are generated by re-heating and slow cooling. In the production process, however, it is hard to control precisely the conditions during the rapid cooling, and it is also hard to control precisely the amount, grain size, distribution and the like of the precipitates produced by melting and decomposing. In addition, the product must be treated further under a grading temperature condition in order to obtain an oriented structure. Thus, the production process results in an increasing man-hour requirement.
Further, when producing a superconductor wire with a silver sheath, a thermal treatment is required at a temperature of the melting point of silver (i.e., 960.degree. C.) or less. However, in the case of YBa.sub.2 Cu.sub.3 O.sub.7-x, the grain boundaries are generated when it is heated and sintered at around 960.degree. C., and no high "Jc" value can be obtained accordingly. On the other hand, when a copper sheath is used, the heating and sintering temperature can be raised to 1000.degree. C. securely. However, since oxygen does not permeate through the copper sheath, and since copper takes oxygens from the oxides disposed in the copper sheath, there occurs an oxygen insufficiency in the copper sheath. Therefore, the copper or copper alloy sheath has not been used recently, and the expensive silver sheath is inevitably used at present. Thus, a degree of freedom is very limited in the selection of an oxide superconductor.